Chromium 49 switched to using SyncToken, making our old model based on
sync-points obsolete. This patch rewrites our syncing to instead take
advantage of new Chromium API for waiting on sync tokens. It also
moves the creation of the GLFences we use out of Chromium.
Task-number: QTBUG-51173
Change-Id: I04d726d4bc81bf6b7fe39bb2b5507e84a0b6991e
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>